1. Field of the Invention
The present invention relates to a liquid crystal display device used in TV receivers, monitors, etc., a substrate for such a liquid crystal display device, and a manufacturing method of such a substrate.
2. Description of the Related Art
In general, a liquid crystal display device is composed of two substrates each having a transparent electrode(s) and a liquid crystal layer that is interposed between the two substrates. The display is controlled by driving the liquid crystal by applying voltages between the transparent electrodes and thereby changing the light transmittance. In recent years, the demand for liquid crystal display devices has increased and requirements for liquid crystal display devices have diversified. In particular, improvement in viewing angle characteristic and display quality is desired strongly and the vertically aligned (VA) liquid crystal display device is considered promising as a means for realizing such requirements.
The VA liquid crystal display device is characterized in that vertical alignment films are formed on the opposite surfaces of two substrates and a liquid crystal layer having negative dielectric anisotropy is interposed between the two substrates. In the VA liquid crystal display device, linear domain regulating means (protrusions or slits) are provided on the two substrates and domain division is performed by the domain regulating means. With these measures, the VA liquid crystal display device attains superior viewing angle characteristic and display quality.
At present, the interval (cell thickness) between the two substrates of a liquid crystal display device is kept by means of spherical or rod-shaped spacers made of plastics or glass. Usually, the spacers are sprayed on one of the two substrates in a spacer spraying process that is executed before attaching the substrates. Then, the two substrates are attached with each other and pressed against each other so that the cell thickness is kept close to the diameter of the spacers.
FIG. 35 is a plan view showing the configuration of a conventional VA liquid crystal display device, and specifically shows three pixels in which color filters of R (red), G (green), and B (blue) are arranged in order on a color filter (CF) substrate 106. A light shield film (BM) is formed on the CF substrate 106 so as to extend in the top-bottom direction and the right-left direction in FIG. 35 and thereby defines individual pixel regions. Protrusions 102 as linear alignment regulating structures are formed in each pixel region obliquely with respect to its end portions. On an array substrate (not shown in FIG. 35) that is opposed to the CF substrate 106, linear protrusions 103 are formed obliquely with respective to the sidelines of each pixel region so as to be deviated from the protrusions 102 by a half pitch. FIG. 35 does not show a storage capacitor bus line that traverses pixel regions at the center.
FIG. 36 is a sectional view of the VA liquid crystal display device taken along line C—C in FIG. 35, and shows a state of a liquid crystal LC when no voltage is applied. The array substrate 104 has pixel electrodes 110 that are formed on a glass substrate 108 in the respective pixel regions. The protrusions 103 are formed on the pixel electrodes 110. A vertical alignment film (not shown) is formed on the entire surfaces of the pixel electrodes 110 and the protrusions 103. On the other hand, the CF substrate 106 has a BM that is formed on a glass substrate 108. The color filters R, G, and B are formed in the respective pixel regions that are defined by the BM that is formed on the glass substrate 108. A common electrode 112 is formed on the color filters R, G, and B and the protrusions 102 are formed on the common electrode 112. A vertical alignment film (not shown) is formed on the entire surfaces of the common electrode 112 and the protrusions 102. The liquid crystal LC is sealed between the array substrate 104 and the CF substrate 106.
As shown in FIG. 36, liquid crystal molecules (indicated by cylinders in FIG. 36) are oriented approximately perpendicular to the substrates 104 and 106. The liquid crystal molecules in the regions where the protrusions 102 and 103 are formed are oriented approximately perpendicular to the surfaces of the protrusions 102 and 103 and are slightly inclined against the substrates 104 and 106. Since polarizers (not shown) are disposed outside the respective substrates 104 and 106 in the crossed Nichols state, black display is obtained when no voltage is applied.
FIG. 37 is a sectional view, similar to FIG. 36, of the VA liquid crystal display device taken along line C—C in FIG. 35, and shows a state of the liquid crystal LC when voltages are applied. Broken lines indicate electric field lines extending between the pixel electrode 110 and the common electrode 112. As shown in FIG. 37, when a voltage is applied between the pixel electrode 110 and the common electrode 112, the electric field is distorted near the protrusions 102 and 103 which are made of a dielectric, whereby the inclination direction of liquid crystal molecules having the negative dielectric anisotropy is regulated. Gradation display can be attained by controlling the inclination angle in accordance with the electric field strength.
Where the protrusions 102 and 103 are formed in a linear manner as shown in FIG. 35, when a voltage is applied, the liquid crystal molecules in the vicinity of the protrusions 102 and 103 fall in the two directions that are perpendicular to the extending direction of the protrusions 102 and 103, with the protrusions 102 and 103 being as boundaries. Since the liquid crystal molecules in the vicinity of the protrusions 102 and 103 are slightly inclined from the direction perpendicular to the two substrates 104 and 106 even when no voltage is applied, they fall quickly in response to electric field strength. The inclination directions of the liquid crystal molecules around the above liquid crystal molecules are determined in order according to the behavior of the above liquid crystal molecules, and the liquid crystal molecules around the above liquid crystal molecules fall in accordance with the electric field strength. In this manner, the domain division is realized in which the protrusions 102 and 103 as the alignment regulating structures serve as boundaries.
Incidentally, FIGS. 36 and 37 do not show spacers that determine the cell thickness. How spacers are arranged will be described with reference to FIG. 38, which is a sectional view taken along line A—A in FIG. 35. Together with the liquid crystal LC, spacers 114 for maintaining the cell thickness between the array substrate 104 and the CF substrate 106 are sealed between the two substrates 104 and 106.
In the VA liquid crystal display device shown in FIG. 38, the protrusions 102 and 103 are formed on the two substrates 104 and 106. Therefore, the cell thickness is determined in one case by spherical spacers 114 that are placed on a protrusion 102 or in another case by spacers 114 that are not placed on any protrusion 102. As such, it is difficult to obtain a uniform cell thickness distribution. To obtain a uniform cell thickness distribution, it is preferable that the number of asperities on the surface of the two substrates 104 and 106 is as small as possible.
FIG. 39 is a sectional view, taken along line C—C in FIG. 35, of a VA liquid crystal display device that is obtained by replacing the protrusions 103 on the array substrate 104 with slits 118 in the VA liquid crystal display device of FIG. 35, and shows a state of the liquid crystal LC when a voltage is applied. As shown in FIG. 39, in the regions where the slits 118 are formed, approximately the same electric field lines are formed as in the regions in FIG. 37 where the protrusions 103 are formed. In this manner, domain division in which the protrusions 102 and the slits 118 serve as boundaries is realized.
FIG. 40 is a sectional view taken along line A—A in FIG. 35, of the VA liquid crystal display device that is obtained by replacing the protrusions 103 on the array substrate 104 with the slits 118 in the VA liquid crystal display device of FIG. 35, and shows a state of the liquid crystal LC when a voltage is applied. As shown in FIG. 40, the liquid crystal molecules in end portions (circled in FIG. 40) of the pixel electrode 110 fall in the different direction than nearby liquid crystal molecules. The light transmittance is low in these alignment defective regions and the presence of these regions is a factor of lowering the luminance in white display.
FIG. 41 is a plan view showing the configuration of still another conventional VA liquid crystal display device, and specifically shows three pixels of R, G, and B on the CF substrate 106. The components in FIG. 41 having the same functions as the corresponding components in FIG. 35 are given the same reference symbols as the latter and will not be described. The liquid crystal display device shown in FIG. 41 is characterized in that slits 118 are formed in place of the protrusions 103 on the array substrate 104, and that auxiliary protrusions 116 that branch off the protrusions 102 and extend along the end portions of each pixel region that extend in the top-bottom direction in FIG. 41 are formed on the CF substrate 106. Although not shown in FIG. 41, the slits 118 have connecting portions, whereby the divided portions of the pixel electrode of each pixel are electrically connected to each other. Together with the protrusions 102, the auxiliary protrusions 116 function as alignment regulating structures. The protrusions 102 determine the viewing angle characteristic of the liquid crystal display device and the auxiliary protrusions 116 control liquid crystal alignment defects due to electric fields that develop in the vicinity of the end portions of each pixel electrode 110. FIG. 41 does not show a storage capacitor bus line that traverses pixel regions at the center.
FIG. 42 is a sectional view of the VA liquid crystal display device taken along line D—D in FIG. 41. In the liquid crystal display device shown in FIG. 42, the slits 118 is formed on the array substrate 104 in place of the protrusions 103 and the auxiliary protrusions 116 are formed on the CF substrate 106. As shown in FIG. 42, the auxiliary protrusions 116 eliminate the alignment defects that occur in the end portions of each pixel electrode 110 (circled in FIG. 40).
Incidentally, the protrusions 102 and the auxiliary protrusions 116 are different in the necessary alignment regulating force. The alignment regulating force of the protrusions 102 is desired to be strong because it determines the liquid crystal alignment direction when voltage is applied. On the other hand, the alignment regulating force of the auxiliary protrusions 116 is desired to be well balanced with the electric fields that develop in the vicinity of the end portions of each pixel electrode 110. FIGS. 43A to 43C show how liquid crystal molecules are oriented in an end portion of each pixel electrode 110. The protrusion 102 is formed on the CF substrate 106 obliquely with respect to the end portion of the pixel electrode 110 and the slit 118 is formed on the array substrate 104 obliquely with respect to the end portion of the pixel electrode 110. The auxiliary protrusion 116 is further formed on the CF substrate 106 so as to branch off the protrusion 102 and extend along the end portion of the pixel electrode 110 that extends in the top-bottom direction in FIGS. 43A to 43C.
FIG. 43A shows a state that the alignment regulating force of the auxiliary protrusion 116 is balanced with the electric field that develops in the vicinity of the end portion of the pixel electrode 110. Because of the alignment regulating force of the auxiliary protrusion 116, the liquid crystal molecules in the end portion of the pixel electrode 110 are oriented approximately parallel with the other liquid crystal molecules, whereby the light transmittance is increased and no reduction in luminance occurs. FIG. 43B shows a state that the alignment regulating force of the auxiliary protrusion 116 is weak. The alignment defect of the liquid crystal molecules in the end portion of the pixel electrode 110 is not controlled sufficiently, and the liquid crystal molecules in the end portion of the pixel electrode 110 are not oriented parallel with the other liquid crystal molecules. No improvement is attained in the problem of reduction in luminance in a region that is hatched in FIG. 43B. FIG. 43C shows a state that the alignment regulating force of the auxiliary protrusion 116 is too strong. Because of the alignment regulation by the auxiliary protrusion 116, the liquid crystal molecules in the end portion of the pixel electrode 110 are oriented approximately perpendicular to the auxiliary protrusion 116. In this state, the alignment direction of the liquid crystal molecules approximately coincides with the absorption axis of the polarizer and hence no improvement is attained in the problem of reduction in luminance in a region that is hatched in FIG. 43C.
In general, the liquid crystal alignment regulating force of an alignment regulating protrusion becomes stronger as it is made higher or wider. That is, to obtain a stable liquid crystal alignment state, it is desirable to form high and wide alignment regulating protrusions. However, if high and wide alignment regulating protrusions are formed, the degree of undulation of the substrate surfaces becomes high and it becomes difficult to obtain a uniform cell thickness by spraying spacers. Therefore, the conventional VA liquid crystal display device has a problem that it is difficult to obtain a uniform cell thickness distribution while the alignment regulating forces of a plurality of alignment regulating protrusions formed on the substrates are optimized in the respective prescribed regions.